The present invention relates to a method and composition for protecting a meltable substance on a substrate, and more particularly, to an improved removable solder stop composition and method for protecting solder on a printed circuit or other substrate during the application of heat, such as during a solder dipping procedure.
In the manufacture of various modules, such as printed wiring boards and similar devices, it is frequently necessary to protect a meltable substance, such as solder deposited on a substrate, from heat to prevent remelting of the substance. For example, printed wiring boards utilize utectic tin-lead alloy solder plating which is reflowed to insure post solderability. The reflowed surface is flat and smooth. Frequently, this flatness must be retained on the bottom side of the printed wiring board for grounding/EMI purposes which mate to a flat housing. If this surface is not protected from the heat of molten solder in a solder pot, it will remelt and form uneven surfaces upon contact and exposure to the molten (260.degree. C.) solder in the solder pot. The lumpy and irregular surfaces interfere with the grounding contact to the castings for EMI/RFI shielding. To prevent remelting and the formation of lumpy and/or irregular surfaces on the solder, the solder alloy must be protected in selected areas from the heat (260.degree. C.) of the hot solder in the solder pot during the in-line soldering of the components placed in the printed wiring boards. The surface of the printed wiring board comes into intimate contact with the hot solder for several seconds during the regular soldering operation.
Generally, the in-line soldering of the components is achieved on an in-line soldering machine where the solder stop is applied first, then exposed to pre-heating, followed by soldering with hot molten solder from a solder pot and in-line cleaning of the total assembly. During the cleaning operation, the solder stop must be completely removed.
Solder stops are applied in various ways, however, commonly, solder stops are screened by silk screening techniques over the surfaces of the solder in the areas where it is desired to protect the solder plate from remelting or picking up additional solder from the molten solder in the pot.
Several of the well-known solder stops must be removed by organic solvents which present environmental and health hazards. Water-soluble solder stops are also known in the prior art, however, they are generally inadequate and/or uneconomical when used in protecting the meltable substance, such as solder, on a substrate.
It is generally desirable that solder stops be water-soluble, or at least, removable from the substrate to which they are applied, by water. It is also preferred that the water soluble solder stop be curable to a dry condition in order to permit handling and shipping as well as in-line machine use. The preferred solder stops must adhere to the surface to which they are applied and must be non-tacky to touch with no cracking or flaking from the substrate. Furthermore, the solder stops must not be affected by the application of flux prior to soldering. Although many types of application systems and methods may be used to apply solder stops to the substrates being protected, it is generally preferred that the solder stop compositions be applied by conventional silk screening techniques and/or stencilling techniques and/or other high speed methods. The cured and/or dried coating must be a good thermal barrier to protect the meltable substrate while the substrate is being subjected to temperatures up to about 260.degree. C. or higher, for example, in a molten solder pot. Finally, the solder stops are removable by water, for example, by in-line water cleaners. The ingredients of the solder stop must not contaminate or leave residues on the printed wiring board or other substrate. The prior art solder stops are incapable of achieving all of the foregoing desirable characteristics.
One of the prior art solder stops contains glycerin and amorphous silica. This prior art composition is water soluble and can be removed by in-line water cleaners. However, the glycerin/amorphous silica composition does not provide a good thermal barrier to protect meltable solder plated circuits while in contact with molten solder at temperature up to 260.degree. C. (500.degree. F.) in a solder pot. Furthermore, the glycerine/amorphous silica solder stop and other prior art products are either tacky, especially when the humidity is high, or they crack and shrink during cold weather and are not sufficiently insulating to prevent the solder surfaces covered therewith from reflowing. The only satisfactory prior art technique is the adhesive masking tape technique which is very costly to apply because it must be done manually and selectively. Furthermore, the adhesive masking tapes frequently bubble and lift from the substrate as a result of the heat from the molten solder in the solder pot. Furthermore, masking tapes must be removed manually from the substrate being protected.